1. Field of the Invention
The present invention relates to an membrane electrode assembly, which is used in a fuel cell and a process for producing the same. More Particularly, the invention relates to an membrane electrode assembly for a fuel cell having electrode catalyst layers and an electrolyte membrane unified with each other, and a process for producing the same.
2. Description of Related Arts
A fuel system is a system mainly composed of a fuel cell, which generates power by supplying hydrogen as fuel gas to a hydrogen pole side of the fuel cell and supplying oxidant gas containing oxygen to an oxygen pole side of the fuel cell. The fuel cell, which is the core part of the fuel cell system, directly converts chemical energy into electric energy, and has recently attracted considerable attention due to its high generation efficiency and little emission of harmful substances, etc.
First, a single cell, which makes up the fuel cell, will be described by referring to FIG. 1.
As shown in FIG. 1, the single cell CE is configured of a membrane membrane electrode assembly MEA composed of an electrolyte membrane M having electrode catalyst layers 1 (1H and 1O) provided on both surfaces (hydrogen pole and oxygen pole sides) thereof, and diffusion layers 2H and 2O and separators 3H and 3O laminated on both sides of the membrane membrane electrode assembly MEA, respectively. Parts residing at the oxygen pole side are expressed herein as numeral with subscript O and parts residing at the hydrogen pole side are expressed herein as numeral with subscript H. Also, if a part is not distinguished, no subscript is referred.
As the electrolyte membrane M, a polymer electrolyte membrane, such as perfluorocarbon sulfonic acid membrane, which is a proton (ion) exchange membrane, is generally used. The electrolyte membrane M has many proton exchange groups in the molecule thereof, exhibits low specific resistance not more than 20 xcexa9/cm at normal room temperature under the moisture saturation conditions, and serves as a proton conductive electrolyte membrane. By utilizing such a polymer electrolyte membrane in the single cell CE of the fuel cell, the fuel cell composed of the lamination of single cells CE is referred to as a polymer electrolyte membrane type fuel cell.
Each of the electrode catalyst layers 1 is composed of catalyst particles having catalytic metal such as platinum (which catalyzes oxidation or reduction) supported on a carrier such as carbon dispersed in an ion (proton) conductive resin.
Each of the diffusion layer 2 is provided in contact with a passage 4 on the surfaced of the separator 3, and has a function of transmitting the electrons between the electrode catalyst layer 1 and the separator 3, and the function of diffusing the fuel gas (hydrogen gas) or the oxidant gas (air) to be supplied into the electrode catalyst layer 1. The diffusion layer 2 is generally formed of a carbon material such as carbon paper, carbon cloth and carbon felt.
The separator 3 is composed of a material excelling in air tightness and excelling in thermal conductivity. The separator 3 separates the fuel gas, the oxidant gas, and the coolant from each other, has a passage 4, and serves as electron-transmitting function.
In the single cell CE of the fuel cell, when supply air is allowed to flow within the gas passage 4O of the separator 3O at the oxygen pole side and supply hydrogen is allowed to flow within the gas passage 4H of the separator 3H at the hydrogen pole side, the hydrogen at the hydrogen pole side is ionized by the canalization of the catalyst in the electrode catalyst layer 1H to produce protons, which are migrated through the electrolyte membrane M and arrived at the oxygen pole side. The protons arrived at the oxygen pole side are readily reacted with the oxygen contained in the supply air in the presence of the catalyst of the electrode catalyst layer 1O to produce water. The supply air containing the produced water and the oxygen remaining unused is discharged from an outlet of the fuel cell FC at the oxygen pole side as exhaust air (which contains a large amount of water). At the hydrogen pole side, electrons, exe2x88x92, are produced during the ionization of hydrogen. The produced electrons, exe2x88x92, are configured to be arrived at the oxygen pole side via an external load such as a motor (arrow in FIG. 1).
Several hundreds of such single cells are laminated to make up a fuel cell and are carried, for example, on a vehicle, etc. to be used.
Conventionally, in the single cell CE of the fuel cell having a configuration as described above, the electrode catalyst layers 1 have been constructed by adhering the electrode catalyst layers 1 onto the electrolyte membrane M, followed by thermally fusing them through a hot press or any other means. However, the electrode catalyst layer 1 has flat interfaces between the electrode catalyst layer 1 and the membrane electrode M, although it possesses several bite portions due the irregularity of the electrode catalyst layer 1.
In the case where the fuel cell is used, for example, in a vehicle, etc., the fuel cell has a hot cycle from the open air temperature (not more than freezing point at a winter season) up to a temperature exceeding approximately 85xc2x0 C. during the course of running the vehicle. Consequently, adhering strength between the electrolyte membrane and the electrode catalyst layer enough for bearing the high temperature environment such as running the vehicle at a high temperature cannot be obtained, sometimes causing exfoliation between the electrolyte membrane and the electrode catalyst layer. In some cases, the interface between the electrolyte membrane and the electrode catalyst layer is aged during such a temperature cycle. Consequently, there leaves something to be improved.
An object of the present invention is, therefore, to provide an membrane electrode assembly for fuel cell, which can prevent from exfoliation of the interface between the electrolyte membrane M and the electrode catalyst layer 1, and which has high durability under temperature cycle from a temperature not more than freezing point up to a temperature not less than approximately 85xc2x0 C.
Another object of the present invention is to provide a process for producing such a highly durable membrane electrode assembly for fuel cell with high efficiency.
We have made serious studies in light of the above situations. As a result, it has been found that these and other objects can be attained when an membrane electrode assembly for fuel cell composed of a pair of electrode catalyst layers and an electrolyte membrane sandwiched between the electrode catalyst layers is configured so that the catalysts of at least one surface of the electrode catalyst layers enter in the electrolyte membrane whereby the electrode catalyst layer and the electrolyte membrane are unified with each other. The present invention has been accomplished based on the discovery.
Specifically, the present invention relates to an membrane electrode assembly for a fuel cell composed of a pair of electrode catalyst layers and an electrolyte membrane sandwiched between the electrode catalyst layers configured so that the catalyst of at least one surface of the electrode catalyst layers enters in the electrolyte membrane whereby the electrode catalyst layer and the electrolyte membrane are unified with each other.
By such a configuration, the electrode catalyst layer or layers and the electrolyte membrane are formed into unification in such a manner that the compositions of the electrode catalyst layer and that of the electrolyte membrane are continuously changed around the interface between them. Consequently, no exfoliation occurs on the interface between the electrode catalyst layer and the electrolyte membrane, and the membrane electrode assembly having improved durability under a prescribed heat cycle can be provided.
In the membrane electrode assembly according to the present invention, it is preferable that an ion exchange density gradient coefficient calculated by the following equation (1):
Coefficient=(Axe2x88x92B)/(Cxe2x88x92D)xe2x80x83xe2x80x83(1) 
wherein A is an ion exchange capacity of the electrolyte membrane, B is an ion exchange capacity of the formed electrode catalyst layer, C is a thickness in xcexcm of the electrolyte membrane before heating under a pressure, and D is a distance in xcexcm of the catalyst particles entering into the electrolyte membrane in xcexcm is not greater than 3.5xc3x97103 meq/g/cm.
By defining the portion where the electrode catalyst layer and the electrolyte membrane are unified with each other, the durability of the membrane electrode assembly becomes much more liable.
In the membrane electrode assembly according to the present invention, the depth of the catalyst entering in the electrolyte membrane is preferably in the range of from 5 xcexcm to 20 xcexcm.
Similarly, by defining the portion where the electrode catalyst layer and the electrolyte membrane are unified with each other, the durability of the membrane electrode assembly becomes much more liable.
According to one aspect of the present invention, the membrane electrode assembly for the fuel cell of the present invention is configured by dissolving the catalyst in a polar solvent and then dispersing the catalyst in an ion conductive polymer solution to prepare a slurry, directly applying the slurry onto at least one surface of the electrolyte membrane, followed by heating under a pressure to form the electrode catalyst layer or layers and to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
Consequently, the present invention is directed to a process for an membrane electrode assembly for a fuel cell composed of a pair of electrode catalyst layers and an electrolyte membrane sandwiched between the electrode catalyst layers configured so that the catalyst of at least one surface of the electrode catalyst layers enters in the electrolyte membrane whereby the electrode catalyst layer and the electrolyte membrane are unified with each other, which comprises:
dissolving the catalyst making up the electrode catalyst layer in a polar solvent and then dispersing the catalyst in an ion conductive polymer solution to prepare a slurry having a viscosity of from 5,000 to 25,000 mPa.s,
directly applying the resulting slurry onto at least one surface of the electrolyte membrane, followed by heating under a pressure to form the electrode catalyst layer or layers, and
heating the slurry under a pressure to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
By such a configuration, a durable membrane electrode assembly can be produced easily and securely. As used herein, the term xe2x80x9cpolar solventxe2x80x9d means any solvent which can exhibit a property of dissolving the electrolyte membrane.
In the process for an membrane electrode assembly for a fuel cell according to this embodiment, it is preferred that the heating under a pressure is carried out in a state where the polar solvent remaining in the slurry directly applied onto the electrolyte membrane is in amount of from 20 mg/cm2 to 100 mg/cm2 to allow the catalysts for entering in said electrolyte membrane.
By such a configuration, a much more durable membrane electrode assembly can be produced easily and securely.
According to another specific aspect of the present invention, the membrane electrode assembly for the fuel cell of the present invention is configured by dispersing the catalyst in a polar solvent soluble in the electrolyte membrane to prepare a slurry of the catalyst dispersed in the organic solvent, directly applying the slurry onto at least one surface of the electrolyte membrane, followed by heating under a pressure to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
Consequently, the present invention is directed to a process for an membrane electrode assembly for a fuel cell composed of a pair of electrode catalyst layers and an electrolyte membrane sandwiched between the electrode catalyst layers configured so that the catalyst of at least one surface of the electrode catalyst layers enters in the electrolyte membrane whereby the electrode catalyst layer and the electrolyte membrane are unified with each other, which comprises:
dispersing the catalyst making up the electrode catalyst layer in an organic solvent soluble in the electrolyte membrane to prepare a slurry of the catalyst dispersed in the organic solvent having a viscosity of from 5,000 to 25,000 mPa.s,
directly applying the resulting slurry onto at least one surface of the electrolyte membrane, followed by heating under a pressure to form the electrode catalyst layer or layers, and
heating the slurry under a pressure to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
By such a configuration, a durable membrane electrode assembly can be produced easily and securely.
In the process for an membrane electrode assembly for a fuel cell according to this embodiment, it is preferred that the heating under a pressure is carried out in a state where the polar solvent remaining in the slurry directly applied onto the electrolyte membrane is in amount of from 20 mg/cm2 to 100 mg/cm2 to allow the catalysts for entering in said electrolyte membrane.
By such a configuration, a much more durable membrane electrode assembly can be produced easily and securely.
According to still another specific aspect of the present invention, the membrane electrode assembly for the fuel cell of the present invention is configured by dissolving the catalyst making up the electrode catalyst layer in a polar solvent and then dispersing the catalyst in an ion conductive polymer solution to prepare a slurry, directly applying the slurry onto at least one surface the diffusion layer to be laminated on both sides of the membrane electrode assembly, applying the electrolyte membrane onto the applied surface of the slurry, followed by heating under a pressure to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
Consequently, the present invention is directed to a process for an membrane electrode assembly for a fuel cell composed of a pair of electrode catalyst layers and an electrolyte membrane sandwiched between the electrode catalyst layers is configured so that the catalyst of at least one surface of the electrode catalyst layers enters in the electrolyte membrane whereby the electrode catalyst layer and the electrolyte membrane are unified with each other, which comprises:
dissolving the catalyst making up the electrode catalyst layer in a polar solvent and then dispersing the catalyst in an ion conductive polymer solution to prepare a slurry having a viscosity of from 5,000 to 25,000 mPa.s,
directly applying the resulting slurry onto at least one surface the diffusion layer laminated on both sides of the membrane electrode assembly, applying the electrolyte membrane onto the applied surface of the slurry, followed by heating under a pressure to form the electrode catalyst layer or layers, and
heating the slurry under a pressure to allow at least parts of the catalysts for entering in said electrolyte membrane during the heating under a pressure whereby said electrode catalyst layer and said electrolyte membrane are unified with each other.
By such a configuration, a durable membrane electrode assembly can be produced easily and securely.
In the process for an membrane electrode assembly for a fuel cell according to this embodiment, it is preferred that the heating under a pressure is carried out in a state where the polar solvent remaining in the slurry directly applied onto the electrolyte membrane is in amount of from 20 mg/cm2 to 100 mg/cm2 to allow the catalysts for entering in said electrolyte membrane.
By such a configuration, a much more durable membrane electrode assembly can be produced easily and securely.